Rechargeable battery becomes inoperable if the buttery deteriorates due to repeatedly charge and discharge. A diagnostic method has been suggested that evaluates the degradation of rechargeable Li-ion battery.
In one diagnostic method for degradation (AC impedance method), impedance of rechargeable battery is measured by measuring voltage and current when alternating voltage or alternating current is applied on the rechargeable battery. Then, impedances are plotted on complex plane when applied frequency is varied from low frequency to high frequency (Cole-Cole plot).
FIG. 13 shows example of Cole-Cole plot. Trajectory of impedance of rechargeable battery at various frequencies is shown in FIG. 13. Degree of degradation of battery can be judged based on difference between the trajectory of impedance of subject battery and the trajectory of impedance of healthy battery
Internal resistance of battery can be measured by measuring voltage change Δ V between during discharge and after stopping discharge. See Shizukuni YATA “High performance storage battery—Part 1, Chapter 2, section 2, performance of discharge and charge and evaluation method of safety of battery” pp 207-213, NTS Co., Ltd.